Hundreds of protesters have clashed with police and security forces in Afghanistan in a second day of angry demonstrations over reports that copies of the Quran were burnt at an airbase used by NATO and coalition troops.

An Afghan government source told Al Jazeera that seven people have been killed during Wednesday's protests and 32 have been wounded, but no exact location was given.

In Kabul, the capital, several people were reported wounded as demonstrators blocked a major highway outside Camp Phoenix, a US base in the city. Police said protesters threw stones, smashed car windows and charged at police lines.

"These protests that began outside the Bagram base on Tuesday, have now seemed to have spread to other cities across Afghanistan."

- Al Jazeera's Bernard Smith

But a police spokesman denied that police officers had shot at protesters.

"People are marching towards Kabul. Police are trying to stop them. We have sent more reinforcements to the area," the spokesman said.

A second protest erupted in west Kabul, involving about 100 university students, a police spokesman said, adding that riot police were present and the demonstration was under control.

According to AFP, one person was killed and 10 were wounded when shots were fired at anti-US demonstrators in the eastern Afghan city of Jalalabad. Protesters there praised the leader of the Afghan Taliban, shouting "Long live Mullah Omar!", Reuters reported.

The US embassy said on Wednesday it was locked down and had suspended all travel in Afghanistan.

US apology

The protests, which followed reports of the discovery by local labourers of charred copies of the Quran as they collected rubbish at the Bagram airbase, prompted apologies from the US government and the commander of NATO-led forces in Afghanistan.

Leon Panetta, the US defence secretary, issued an apology for the "inappropriate treatment" of Islam's holy book at the base and backed General John Allen's call for "swift and decisive action to investigate this matter".

"These actions do not represent the views of the United States military. We honour and respect the religious practices of the Afghan people, without exception," he said.

Al Jazeera's Bernard Smith reporting from Herat said: "These protests that began outside the Bagram base on Tuesday, have now seemed to have spread to other cities across Afghanistan."

Hundreds of protesters besieged Bagram, about 60km north of Kabul, on Tuesday, firing slingshots and petrol bombs at the gate of the base.

Quran copies 'not burnt'

Reports suggest US personnel had confiscated materials that they suspected Taliban prisoners were using to send messages.

Carsten Jacobson, a spokesman for the NATO-led international force in Afghanistan, said an investigation had been launched into the issue and preliminary information showed that Quran copies had not been burned.

"Fortunately for all of us, local workers recognised the type of material and intervened. Actually the disposal process was stopped in time but it led to protests over the day. As far as we know, and the investigations are ongoing, they were not burned. But we have to wait for the results."

Carsten Jacobson, spokesman for the NATO-led force, said the "disposal process" was stopped in time.

Allegations that NATO troops working inside the base had set fire to copies of the Quran were first reported by a senior government official.

“It is surprising that after all these years American and NATO forces have been here in Afghanistan and all the lessons they have learned about how important it is to treat Islamic material with due respect, this sort of thing is still happening," our correspondent said.

"That is what causes so much offence here in Afghanistan and adds fuel to the anti-American and anti-foreigner feelings."